The present invention relates to fuel pumps and, more particularly, to fuel pumps and rail systems for supplying fuel at high pressure for injection into an internal combustion engine.
Current gasoline direct injection systems have a relatively low overall pumping efficiency because, e.g., they employ a constant output pump that is sized for the maximum required output. The excess fuel pressurized by the pump passes through a dumping type pressure regulator and is subsequently returned to the pump inlet or the fuel tank. As the fuel passes through the pressure regulator, the fuel depressurizes releasing energy in the form of heat. Accordingly, a significant amount of energy is wasted pressurizing unused fuel.
In a typical direct fuel injection system, a high-pressure (up to 120 bar) supply pump is employed which pressurizes fuel received from a low-pressure circuit (2 to 4 bar) including, e.g., a fuel tank and a low-pressure fuel pump. An accumulator is typically fluidly connected to the high-pressure pump and fuel regulators are fluidly connected to the accumulator.
The accumulator provides a reservoir of fuel that is pressurized by the pump. The accumulator has to fulfill two main tasks: First it subsidizes the pump output during the injection event, enabling the injection system to inject fuel at a rate higher than the pumping rate and second to attenuate pressure pulsation caused by the instantaneous pumping rate variation as well as by pressure waves created by abrupt fuel velocity changes during opening and closing of the injectors.
The rail volume is a compromise between two contradictory requirements. On the one hand a relatively large accumulator volume is desirable to minimize the pressure drop during the injection event (caused by withdrawal of a fuel amount larger than supplied by the pump) and also to provide a high degree of pressure pulsation attenuation in order to enable the electronics to assess the average pressure in the rail, necessary for calculation of the correct injection duration and also to insure a more or less uniform injection rate. If for example injection pressure would drop substantially during the injection, the fuel amount metering accuracy, atomization and also droplet penetration into the combustion chamber where the pressure already started to rise due to combustion of the initially injected fuel would adversely affecting engine performance and emissions.
On the other hand, it would be desirable the keep accumulator volume relatively small to accelerate pressure transients, especially at low speed, where the pump output over time is the lowest.
During extreme low temperature start conditions (xe2x88x9230 to xe2x88x9240 C.) substantially more fuel has to be injected as not all fuel droplets remain airborne and evaporate before the spark plug is triggered and also relatively high injection pressure is necessary to provide sufficiently fine atomization.
However, during such cold start conditions the cranking speed is very likely to be lower than at higher temperature, partly because of higher viscosity of engine lubricants causing higher resistance against turning and partially because of reduced capacity of the electric battery.
Because of that an accumulator optimized for operation between idle and rated speed under xe2x80x9cnormalxe2x80x9d temperature could be too large during low speed cold cranking conditions, extending the cranking time or even compromising the starting altogether.
Accordingly, it is desirable to reduce the quantity of fuel during cranking necessary to increase of the pressure in the rail by reducing the accumulator volume.
In accordance with one embodiment of the present invention, a self-regulating direct injection fuel delivery system for a motor vehicle includes a common rail that has an accumulator which includes a relatively large fuel volume. The accumulator is connected in fluid communication with a distributor that has a relatively small fuel volume and at least one fuel injector nozzle is connected in direct fluid communication with the distributor. A high pressure pump delivers fuel to the common rail and flow control means are interposed between the pump and the common rail for selectively delivering fuel to one of the accumulator and the distributor and then the other of the accumulator and the distributor.
In accordance with a particular embodiment of the present invention, the flow control means controls both a first flow path between the pump and the distributor and a second flow path between the pump and the accumulator. A pressure control valve is situated in a third flow path between the accumulator and the distributor. The pressure control valve prevents flow from the distributor to the accumulator but permits flow from the accumulator to the distributor when the pressure in the accumulator exceeds the pressure in the distributor by a predetermined differential.
In accordance with another particular embodiment, the pump has an inlet and a discharge, and the flow control means comprises a supply flow path wherein the pump discharge is selectively connected in fluid communication with the first flow path or the second flow path. A bypass flow path may also be provided wherein the pump discharge is selectively connected in fluid communication with the pump inlet.
In accordance with further particular embodiments, the flow control means comprises a control valve for aligning the pump discharge with the first flow path, the pump discharge with the second flow path, and the pump discharge with the bypass flow path. The control valve may comprise a first operator disposed within the first flow path and a second operator cooperatively engageable with the first operator and being disposed within the second flow path. At less than a first predetermined pressure, the second operator is biased into engagement with the first operator so that the first operator aligns the pump discharge with the first flow path only. At greater than the first predetermined pressure, the second operator is urged away from engagement with the first operator thereby aligning the pump discharge with the second flow path. Once a second predetermined pressure is exceeded, the second operator moves to a location wherein the pump discharge is aligned with the bypass flow path.
In accordance with another embodiment of the present invention, a split rail fuel injector assembly for a motor vehicle including a high pressure fuel pump for delivering fuel to at least one fuel injector nozzle is provided. The split rail fuel injection system comprises a distributor for distributing fuel having a distributor first inlet, a distributor second inlet and a distributor outlet. The distributor first inlet is connected in fluid communication with the fuel pump and to the at least one fuel injector nozzle and has a distributor internal volume. An accumulator configured to receive fuel from the fuel pump and to selectively pass fuel to the distributor via the distributor second inlet is provided. The accumulator has an accumulator internal volume wherein the distributor internal volume is substantially less than the accumulator internal volume.
In accordance with a further embodiment of the present invention, a common rail fuel injection system assembly for a motor vehicle includes a high pressure fuel pump that has an inlet and a discharge for delivering fuel to at least one fuel injector nozzle. The injector assembly comprises an accumulator connected in fluid communication with the fuel pump, the accumulator having an accumulator internal volume for containing a reservoir of fuel. Flow control means are interposed between the pump and the accumulator for selectively delivering fuel to the accumulator. The flow control means comprises a supply flow path wherein the pump discharge is selectively aligned with the accumulator and a bypass flow path wherein the pump discharge is selectively aligned with the pump inlet.
In accordance with another embodiment of the present invention, a common rail fuel injection system for a motor vehicle includes a high pressure fuel pump that has an inlet and a discharge for delivering fuel to at least one fuel injector nozzle and a common rail which includes an accumulator connected in fluid communication with a distributor. The fuel injection assembly comprises a flow control device interposed between the pump and the common rail for selectively delivering fuel to one of the accumulator and the distributor and then the other of the accumulator and the distributor. The flow control device controls both a first flow path between the pump and the distributor and a second flow path between the pump and the accumulator. The flow control device comprises a supply flow path wherein the pump discharge is selectively connected to the first flow path or the second flow path and a bypass flow path wherein the pump discharge is selectively connected to the pump inlet. A control valve is provided for selectively aligning the pump discharge with the first flow path, the pump discharge with the second flow path, and the pump discharge with the bypass flow path. The control valve comprises a first operator disposed within the first flow path and a second operator cooperatively engageable with the first operator and being disposed within the second flow path. At less than a first predetermined pressure, the second operator is biased into engagement with the first operator so that the first operator aligns the pump discharge with the first flow path. At greater than a predetermined pressure, the second operator is urged away from engagement with the first operator thereby aligning of the pump discharge with the second flow path. Once a second predetermined pressure is exceeded, the second operator moves to a location wherein the pump discharge is aligned with the bypass flow path.
The invention in another embodiment, is a method of supplying fuel to a plurality of fuel injection nozzles at a target delivery pressure in a distributor rail fluidly connected to each of the nozzles, including the steps of maintaining fuel at a pressure above the target delivery pressure in an accumulator having a volume greater than the volume of the distributor rail; maintaining a differential pressure between a higher pressure in the accumulator and the target pressure in the distributor rail, through a fluid connection between the accumulator and the distributor rail; whereby as pressure in the distributor rail begins to drop when the nozzles inject fuel, fuel at the higher pressure of the accumulator flows into the distributor rail to maintain the target pressure therein. The method preferably includes measuring the pressure in the distributor rail, and responsive to the measured pressure and the target pressure in the distributor rail, controlling a variable position valve fluidly connected between the accumulator and the distributor rail to control the fuel flow into the distributor rail.